This invention relates generally to induction heating arrangements and, more particularly, to power circuits for induction cooking appliances. Induction cooking is attractive for domestic use because the use of induction heating to heat a cooking utensil is a theoretically efficient process. Heat is generated only in the metallic utensil where it is wanted. The ordinary gas range and electric range, by comparison, have greater losses due to poor coupling of heat to the utensil and heating the surrounding atmosphere. However, the frequency of power supplies available in the home, generally on the order of 50 to 60 Hz, is too low to effectively drive an induction heating coil. For cooking appliances, ultrasonic frequencies on the order of 20 KHz provide much more satisfactory results.
Inverter circuits commonly employed in induction cooking appliances to convert the 60 Hz line voltage to the high-frequency signal for driving the induction coil use filtered LC resonant circuits to provide the desired ultrasonic frequency signal. Examples of such inverter circuits may be found in U.S. Pat. No. 3,781,503 to Harnden, Jr. et al and U.S. Pat. No. 4,074,101 to Kiuchi et al. In the LC resonant circuits used in such resonant circuit inverters, the effective inductance becomes quite large due to the induction heating coil of the power circuit. Because of this large inductance a large capacitance is required. In addition to the capacitance required for the resonant circuit, a smoothing capacitor is required to filter the rectified line voltage. This capacitor also in necessarily large. The large capacitors required for such circuits are expensive to the extent that the cost of the capacitors becomes the dominant cost of the power circuit.
Alternatives to resonant circuit inverters include inverter circuits of the push-pull type which typically include a transformer having a center tap winding with the ends of the primary windings connected to switching devices driven in push-pull. Such push-pull arrangements conventionally employ a feedback circuit including a feedback transformer to sense load current and control the frequency of the switching device in accordance with sensed load current. Examples of such inverter circuits are disclosed in U.S. Pat. No. 3,383,582 to John D. Bishop et al and U.S. Pat. No. 3,973,165 to Thomas Eugene Hester. The feedback transformer and other circuit elements employed in such feedback circuitry are prohibitively expensive, making such circuits relatively unattractive for cooking appliance applications.
In view of the shortcoming of the prior art, it is apparent that there is a need for a power circuit for an induction cooking appliance which eliminates the costly capacitance associated with the resonant circuit inverters and which eliminates the costly transformer feedback arrangements associated with push-pull inverters of the prior art.
It is therefore an object of the present invention to provide an induction heating arrangement in which an induction heating coil is energized by a pulsating power supply and in which energization of the coil is controlled by switch means operative to cause the direction of current in the induction heating coil to oscillate at a frequency which is independent of load current.
It is a further object of the present invention to provide an induction heating arrangement of the above mentioned type in which the switch means is operative to cause the direction of current in the induction coil to oscillate at a frequency which varies directly with the amplitude of the power supply.
It is a further object of the present invention to provide an induction heating arrangement of the above mentioned type in which the energy delivered to the coil with each current pulse is uniform from pulse to pulse.
It is a further object of the present invention to provide an induction heating arrangement of the above mentioned type which eliminates the need for large costly capacitors and costly transformer feedback circuitry.